For at least 10 years, the transport by sea of natural gas in the liquid form has entered an industrial era. At the present time, there exists an important world trade in natural liquefied gas, in which this source of energy is liquefied in the producer countries, conveyed by sea by means of ships specially designed for that purpose to the consumer countries, and finally vaporized in the latter countries so as to be determined towards the various points of utilization.
This sea transport is carried out by means of methane ships comprising reception tanks for the liquefied natural gas, thermally insulated in such manner as to be able to keep this cargo under a pressure in the vicinity of atmospheric pressure and at temperatures in the neighborhood of -160.degree.C.
The thermal insulation employed is of course not perfect, and a small part of the cargo necessarily becomes vaporized during the course of storage in the methane ship. Furthermore, another part of the liquefied gas becomes vaporized voluntarily or accidentally during the exploitation cycle of the methane ship, for example during the return of the empty ship to a producer country by maintaining the tanks under cold by vaporization of liquefied natural gas along their walls, or during the loading of the cargo of liquefied gas.
By way of example, a methane ship carrying 125,000 cu.m. of liquefied natural gas loses about 0.25% of its cargo per day. For this same ship, 2% of the volume carried is lost for an outward and return journey, each of 4 days.
Under normal conditions of utilization of methane ships, the quantities of vaporized natural gas are thus not negligible, and the problem thus arises of recovering these evaporations, composed in general of methane with possibly a small proportion of nitrogen.
Up to the present time, the natural gas vaporized on board the ship was either discharged to the atmosphere or burned in the boilers of the ship. This solution is becoming less and less satisfactory for the following reasons. On the one hand, in view of the shortage of natural gas and its increasingly high cost, it is becoming prohibitive to definitely lose these evaporations or to burn them in the boilers when other fuels are better suited or less expensive. On the other hand, in view of the increasingly high cost of liquefied natural gas, resulting from the production, transport and delivery of this source of energy, it is becoming imperative to make the necessary capital investments more profitable, by increasing the carrying capacity of the methane ships for the available volume of storage.
For all these reasons, it has now become economically very advantageous to liquefy the evaporations arising from the storage of liquefied natural gas on board the methane ship. The cost of liquefying apparatus utilized is low compared with the gain obtained by increasing the carrying capacity of the methane ship.